In power amplifiers used to transmit modulated radio frequency signals, is desirable to operate with an input signal having a low peak-to-average ratio. Signals with high peak-to-average ratios are undesirable because the power amplifier produces extraneous side bands when a peaking signal causes it to operated in a nonlinear portion of its operating range. These extraneous side bands are produced by a mechanisms called AM-to-PM conversion and AM-to-AM conversion when passing a signal with large amplitude fluctuations. Furthermore, these side bands deprive the information signals of some of their portion of the transponder power, and also can interfere with nearby channels (adjacent channel interference).
In a communications system using quartenary phase shift keying (QPSK) the signal phase can be any of one of four phases for the duration of each phase shift interval. This is shown in the signal space diagram in FIG. 1, wherein phase 30 illustrates the phase of constellation point 32, which is one of the constellation points 32-38. Transitions 40-46 illustrate the permitted phase changes between phase shift intervals. A zero degree transition is shown at reference numeral 40. Examples of .pi./2 radians or 90.degree. transitions are shown at reference numerals 42 and 44, and a 180.degree. or .pi. radian transition is shown at reference numeral 46.
In a code division multiple access (CDMA) system, such as a CDMA system implemented according to American National Standards Institute (ANSI) J-STD-008, user data is spread and modulated by a pseudorandom noise (PN) sequence, which is periodic and has noise-like properties. For example, with reference to FIG. 2, in direct sequence QPSK transmitter 60, real-valued user data 62 is split and multiplied by 2 PN sequences: a PN.sub.I sequence 64 and a PN.sub.Q sequence 66, using multipliers 68 and 70, respectively. The PN sequences are generated by PN.sub.I and PN.sub.Q sequence generators 72 and 74, respectively. The duration of the output of these PN sequence generators may be referred to as a chip time or chip interval, which is the duration of a single pulse in a direct sequence modulated signal.
After in-phase (I) and quadrature (Q) components of user data 62 have been multiplied by PN.sub.I sequence 64 and PN.sub.Q sequence 66, the signals output by multipliers 68 and 70 are each separately filtered by pulse shaping filters 76. Pulse shaping filters 76 may be implemented with finite impulse response filters that filter higher frequency components from the signal.
Next, the filtered I and Q signal components are multiplied by quadrature carrier components 78 and 80 using multipliers 82 to produce I and Q radio frequency (RF) signals 84 and 86. Signals 84 and 86 are then added together in summer 88. The output of summer 88 is RF modulated signal 90, which is then amplified by power amplifier 92. The output of power amplifier 92 is then coupled to antenna 94 for transmitting the signal to a receiving unit.
As shown in FIG. 2, PN sequence generators 72 and 74 are typically implemented with a maximal-length linear feedback N-bit shift register, wherein selected stages are tapped and exclusive ORed with the shift register output to form a signal that is fedback to the shift register input. Other ways of implementing PN sequence generators may be used. For example, nonlinear feedback shift registers may be used to generate the PN sequences.
A combination of the outputs of PN.sub.I and PN.sub.Q generators 72 and 74 may be referred to as having a complex value that corresponds to a phase. For example, referring again to FIG. 1, if PN.sub.I equals 1 and PN.sub.Q equals 1 the complex PN value of (1, 1) corresponds to phase 30, which is .pi./4 radians. Other values output by the complex PN generator correspond to constellation points 34-38. Transitions 40-46 from one constellation point to another are determined by the difference between a previous complex PN chip and a next complex PN chip generated by the complex PN sequence generator in the next chip time.
When RF modulated signal 90 peaks and causes power amplifier 92 to operate in a non-linear region, extraneous side bands are created in the transmitted signal. These side band signals may be eliminated by reducing the occurrence of peaks in RF modulated signal 90, hence the desirability of reducing the peak-to-average ratio.
Peaks in RF modulated signal 90 occur as a result of receiving a sequence of chip values in pulse shaping filter 76 that highly correlates with the impulse response of pulse shaping filter 76. Furthermore, the peaking of signal 90 is greater when peaks are formed in pulse shaping filters 76 in both the I and Q channels at the same time.
In the prior art, .pi./2 BPSK modulation has been used to reduce the peak-to-average in signals sent to the power amplifier. However, .pi./2 BPSK modulation produces BPSK spreading, which is inferior because signals from other users are not easily rejected.
QPSK spreading, on the other hand, provides superior rejection between user's signals, but produces a signal with an inferior peak-to-average ratio. For a more detailed discussion regarding spreading methods, see the book "CDMA, Principles of Spread Spectrum Communications," by Andrew J. Viterbi, published by Addison Wesley in 1995, pages 26-32.
Thus, it should be apparent that a need exists for an improved method and system for generating a complex pseudonoise sequence for processing a code division multiple access signal wherein the complex pseudonoise sequence helps reduce the peak-to-average ratio of a modulated communications signal.